1. Field of the Invention
The present invention relates to a closed cassette and method for using the closed cassette to heat treat (compact) one or more glass sheets in a uniform manner and in a clean environment.
2. Description of Related Art
Manufacturers of glass sheets (e.g., liquid crystal display (LCD) glass sheets) often heat treat the glass sheets to pre-shrink them so they will not shrink or shrink very little when their customers process the glass sheets. Today there are several systems/processes the manufacturers use to heat treat the glass sheets. Three of these systems/process and their associated problems are briefly described below with respect to FIGS. 1-3.
First, the manufacturers can use as shown in FIG. 1 a re-circulating air oven 100 to heat treat glass sheets 112. The re-circulating air oven 100 circulates unfiltered air 102 around one or more large steel muffles 104 where in this example only two muffles 104 are shown one of which is located on a top shelf 106 and the other muffle 104 is located a bottom shelf 108. Each muffle 104 contains multiple open cassettes 110 where in this example each muffle 104 contains fifteen open cassettes 110 of which only five of the open cassettes 110 can be seen on each of the shelves 106 and 108 shown in the side view of FIG. 1. And, each open cassette 110 supports multiple glass sheets 112 where in this example each open cassette 110 is shown to support six glass sheets 112. The muffles 104 are used to isolate the open cassettes 110 from the unfiltered air 102 flowing in the re-circulating air oven 100. The main problems associated with this system/process is that (1) long thermal cycles are needed to heat and cool the glass sheets 112 and (2) the heat/cold transfer and temperature uniformity within each muffle 104 is poor.
Second, the manufacturers can use as shown in FIG. 2 a direct convection oven 200 to heat treat glass sheets 206. The direct convection oven 200 circulates air 202 that has been filtered by one or more high efficiency particulate (HEPA) filters 203 (e.g., one HEPA filter 203) around multiple open cassettes 204 each of which supports multiple glass sheets 206 (e.g., six glass sheets 206). In this example, the direct convection oven 200 supports thirty open cassettes 204 of which only five open cassettes 204 can be seen on a top shelf 208 which is shown in the side view of FIG. 2. And, five open cassettes 204 can be seen on a bottom shelf 210 which is shown in FIG. 2. The direct convection oven 200 uses filtered air 202 to uniformly heat and cool the glass sheets 206. The main problem associated with this system/process is that the HEPA filter 203 has a maximum operating temperature of about 350° C. when the glass sheets 206 need to be heated up to 650° C. to be properly heat treated. It should be appreciated that the system/process and in particular the direct convection oven 200 shown in FIG. 2 is simply a conceptual drawing that is used to help describe a problem associated with using the traditional open cassette 204. As such, this system/process may or may not be used in industry today.
Third, the manufacturers can use as shown in FIG. 3 a radiant oven 300 to heat treat glass sheets 304. The radiant oven 300 radiates heat/cold around one or more open cassettes 302 each of which supports multiple glass sheets 304 (e.g., six glass sheets 304). In this example, the radiant oven 300 supports thirty open cassettes 302 of which only five open cassettes 302 can be seen on a top shelf 306 which is shown in the side view of FIG. 3. And, five open cassettes 302 can be seen located and on a bottom shelf 308 which is shown in FIG. 3. The radiant oven 300 does not use forced convection heating or cooling due to concerns with blowing particles onto the glass sheets 304. As such, the radiant oven 300 simply radiates heat/cold to heat treat the glass sheets 304. The main problem associated with this system/process is that it is difficult for the radiant oven 300 to uniformly heat and cool the glass sheets 304. It should be appreciated that the system/process and in particular the radiant oven 300 shown in FIG. 3 is simply a conceptual drawing that is used to help describe a problem associated with using the traditional open cassette 302. As such, this system/process may or may not be used in industry today.
In view of the problems associated with the traditional systems/processes shown in FIGS. 1-3, there is a need for a system/process that can address the aforementioned shortcomings of the traditional systems/processes by effectively heat treating multiple glass sheets in a uniform manner and in a clean environment. This need and other needs are provided by the convection compatible closed cassette, method and system of the present invention.